During semiconductor processing, it is necessary in the case of certain types of devices to attach a semiconductor die or IC chip onto a carrier such as a leadframe or other substrate. Wire connections must then be made between conductive pads on the die and the carrier respectively so that the die and the carrier are electrically connected. Commonly, an ultrasonic transducer including a capillary attached to the transducer is used to feed bonding wire through the tip of the capillary, such that ultrasonic energy can be transmitted to the tip of the capillary to initiate bonding or welding of the wire to the conductive pad.
During this process of ultrasonic bonding, the carrier must be held securely by a clamping device for high-precision wire-bonding. As the device gets smaller, the challenge of holding each component on the device more securely is increased. Generally, the clamping device should cover and exert pressure on most of the surface area of the carrier since the carrier is usually made of a flexible material that would tend to shift upon the application of forces during a bonding process. The clamping device includes a window over a portion of the carrier that is to be worked upon for access by the capillary of the ultrasonic transducer. Thus the clamping device is often referred to as a window clamp.
It is preferred that the size of the window corresponds as closely as possible to the size of a bonding unit comprising a die and surrounding conductive pads on the carrier so that the maximum area of the carrier is clamped without blocking access to the bonding unit by the capillary and the bonding unit can be held and supported more securely during wire-bonding. In the case of larger dice or if vacuum suction is applicable to assist in holding down the carrier, it is adequate to have a single relatively large window.
However, when the size of the die gets smaller or vacuum suction is not applicable, it is generally not desirable to have an array of bonding units exposed in the window because bonding units or portions thereof that are located away from the edges of the window cannot be clamped securely. Accordingly, the size of the window needs to be correspondingly reduced. Otherwise, the unclamped dice cannot be wire-bonded reliably. On the other hand, to design and manufacture a small window to fit just one small bonding unit would lead to inefficiency and increased cycle time because the window of the clamping device would need to be indexed afresh for each and every bonding unit in order to make wire connections.
One way to overcome this inefficiency is to fabricate a window that comprises many smaller openings divided by support structures so as to increase support for bonding units that are located away from the edge of the window. FIG. 1 is a plan view of a prior art clamping device 100 with multiple elongated slots 108 for access to a electronic device to be processed. The prior art clamping device 100 has a main body 104 and a window 106 located substantially centrally on the main body 104. Each elongated slot 108 corresponds to a position of a row of bonding units, and is separated from an adjacent elongated slot 108 by a rib 110. In this way, the ribs 110 support the bonding units located away from the edges of the window 106, and an array of bonding units may be bonded without having to repeatedly index the bonding units with respect to the clamping device 100.
Unfortunately, in modern high-density devices, the pitches between adjacent bonding units can be very small, such as 1 mm to 2 mm. Moreover, they may not allow for generation of a holding force by vacuum suction. Due to this spacing limitation, it is a formidable challenge to design a clamping device that would support multiple bonding units without adversely affecting the bonding process.
One approach would be to machine ribs 110 with smaller widths in order to match the pitches between adjacent bonding units. There would be various difficulties associated with such an approach. One problem is that there is usually a machining limitation in CNC (Computer Numerical Control) machines with respect to the length-to-width ratio of the ribs. For high-density devices with smaller pitches and a relatively large bonding area, ribs of a certain length have to be made slimmer. Machining is not practicable because as the ribs get slimmer, it is very difficult for them to maintain their linearity, or they may be easily broken because of their lack of rigidity. These factors have an impact on the feasibility and costs of manufacturing such a window clamp, as well as clamping reliability.
Another possible alternative to having to machine multiple elongated slots is to provide a clamping device with a window in the form of a single elongated slot that exposes a row or column of bonding units in the array. FIG. 2 is a plan view of a prior art clamping device 102 with a window 106 comprising a single elongated slot. A width of the slot corresponds to a width of a row or column of bonding units that are to be bonded. Nevertheless, the clamping device would have to be indexed after each row or column of dice are bonded, thereby reducing the bonding speed. The pattern recognition (PR) system of the bonding machine is also unable to look forward to the next row or column to be bonded to perform advance data processing, further resulting in lower output capacity for a given time. It would be desirable to reduce the indexing frequency during bonding and increase the output capacity for a given time.